Convergent Plate Boundaries

When two plates move towards each other a collision takes place. There are two types of plates: oceanic and continental, so there are three types of convergent plate margins: oceanic-continental: collision of the Nazca plate and South American, forming the Andes oceanic-oceanic: the collision of the Pacific plate and the Eurasian plate in Japan continental-continental: the collision of India with Eurasia, forming the Himalayas The plate which subducts may be older and therefore colder, or it may be slower. It is always the oceanic plate which descends because it is made of dense basalt compared to the lighter granitic continental plate.Evidence for Convergent Plate BoundariesThere are many processes operating at plate boundaries that do not occur elsewhere. This means that looking at the distribution of these features allows us to identify modern plate boundaries. We can use the same ideas to identify ancient plate boundaries. The presence of intermediate volcanic rocks, ophiolites, batholiths and folded and faulted rocks forming an ancient fold mountain belt in Scotland tells us that this was an active plate boundary 350 million years ago.Heat Flow AnomaliesHeat flow measurements at both oceanic-oceanic and oceanic-continental margins show the same pattern. There is a negative anomaly at the trench where cold lithosphere is subducting and a positive anomaly at the island arc or volcanic belt where magma is being intruded.Volcanic ActivityAs the subducting plate heats up there is a partial melting along the top surface of the descending oceanic crust, producing a basaltic magma. This is less dense than the surrounding material and so rises to form volcanoes in an island arc or chain in a continent. However, basaltic magma is at a temperature of 1200 degrees and as this rises through silicic continental crust it causes partial melting and some mixing of magma may occur to give intermediate rock, andesite. BatholithsWhere continental crust is melted by magma most of the material will remain separate as the viscosity of the magmas is very different. The melted silicic material can form large granite batholiths deep in the continental crust below fold mountains.TrenchesTrenches form above a point where the oceanic plate is being subducted. They are long, narrow, linear features that form the deepest parts of the Earth's surface, arranged parallel to the edge of the plate.Fold Mountain Belts/Orogenic BeltsFold mountains form on the edges of the continents parallel to the subduction zone. They are compressional features made of folded and faulted sediments that have been scraped off the descending plate onto the non-subducting plate. The high pressures and temperatures at convergent plate margins means that rocks are regionally metamorphosed.Benioff ZoneAs the oceanic plate descends, the sloping plane of the boundary is marked by increasingly deep focus earthquakes along the Benioff Zone. At higher levels in the crust, where subduction is beginning, the earthquakes are generated along the boundary itself partly due to friction. Further down, the foci occur within the plate itself as the interior part of the plate remains colder and more rigid, while the edges of the plate heat up and move more easily. Convergent Continental-Continental Plate MarginsThe continental plates of similar composition and density meet under the Himalayas. The Indian plate has spent the last 100 million years as an island, drifting north away from Africa and Antartica when it was part of Gondwanaland. Movement was initially slow but increased in the last 60 million years, with a maximum rate of 19cm a year until it collided with continental Asia.The smaller Indian plate was forced under the larger Asian plate, producing shallow and intermediate earthquakes along the boundary. However, subduction soon stopped because there is no difference in density, and the plates crumpled together instead. The sediments deposited in the ocean which occupied the decreasing space between the advancing plates were intensely compressed and metamorphosed. They now form the fold mountains of the Himalayas. The remains of the subduction zone sank beneath the collision zone without having the opportunity to form magma to feed volcanoes.The continental crust increased in thickness and its base began to melt, creating viscous, silicic magma which slowly rose through the overlying crust to form granite batholiths. No lava was erupted.The Indian plate is less dense than the surrounding mantle into which it was forced, and the additional buoyancy elevated the mountains - the Himalayas - to a greater average height.

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